The back electromotive force voltage of the stator armature of a brushless DC motor is proportional to the rotational speed of the rotor, but the rotor magnetic field thereof is fixed. Therefore, when the motor is operated to the maximum output of the rated speed, appropriate current phase control is required to obtain the maximum torque. However, since the brushless DC motor is an inductive load, as the speed increases, the phase current will fall behind the phase voltage, thus causing the torque output to deteriorate.
The phase angle of the output voltage of the existing motor needs to be manually adjusted, so that a three-phase DC brushless motor can maintain good use efficiency under different operating conditions. However, since the operating condition for the three-phase DC brushless motor changes significantly according to different applications thereof, the conventional method will cost a lot of manpower and is not effective.
Therefore, how the above-mentioned defects can be overcome by improvement of the circuit design, so that the motor driving circuit can automatically adjust the phase current and the back EMF to be phase synchronized, has become an important issue in the art.